1. Field of the Invention
The present invention relates to a heater for heating an object material, such as a gas, a liquid and the like, and to a glow plug for a diesel engine.
2. Description of the Related Art
A related art heater will be described on the basis of FIG. 6 of the accompanying drawings taking as an example the glow plug disclosed in Japanese Patent Laid-Open No. 10-2557. This glow plug 100 roughly includes a main metal member constituting a tubular member 101, a cap type sheath tube 102 fixed to a front end of the tubular member 101, and a central electrode constituting a shaft-like member 103. The shaft-like member 103 extends through the center of the tubular member 101 to reach the sheath tube 102, and a front end of the sheath tube 102 and that of the shaft-like member 103 are electrically connected to each other via a control coil 104 and a heating coil 105. A stepped bore 106 is formed in a rear end (upper portion of the drawing) of the tubular member 101, and an upper portion of the shaft-like member 103 is fixed in the center of the tubular member 101 by a bush type insulating ring 107 fitted in the stepped bore 106.
Thus, the glow plug 100 employs a seal structure having a clearance 108 between the surface of the tubular member 101 which defines the stepped bore 106 and the shaft-like member 103, an O-ring 109 being fitted in the clearance 108 and crushed by a stepped portion of the stepped bore 106 and the insulating ring 107, the clearance 108 being thereby closed.
3. Problem Solved by the Invention
When the O-ring 109 is inserted into the stepped bore 106 of the glow plug 100, there is the possibility that the O-ring 109 be broken or cracked, i.e., damaged. Especially, in recent years, the miniaturization of the glow plug 100 has progressed, and a glow plug in which a nominal diameter (diameter of a ridge) of a male thread 110 to be engaged with a diesel engine is not larger than 10 mm is being manufactured. In such a glow plug, a stepped bore 106 has a small diameter, and an O-ring 109 is extremely small, so that the O-ring is in great danger of being damaged. Needless to say, the damage to the O-ring 109 causes an imperfect sealing of the glow plug 100 and exerts a bad influence upon the performance thereof. Therefore, it becomes essential to visually inspect the O-ring after the O-ring is inserted into the stepped bore, and this renders it difficult to automate the O-ring setting operation.
The present invention has been made in view of the above-mentioned circumstances, and has as an object to provide a heater and a glow plug which have a seal structure adapted to seal with an O-ring a clearance between an inner surface of a stepped bore in a tubular member and an outer surface of a shaft-like member inserted into the stepped bore, and which are capable of reducing a rate of occurrence of damage to the O-ring during an O-ring setting operation.
The present invention (1) provides a heater having a seal structure formed by inserting a shaft-like member into a stepped bore made in a tubular member, and providing an O-ring on a stepped portion of the stepped bore so as to close a clearance between the shaft-like member and tubular member, wherein a seal region is formed on a stepped portion of the stepped bore, a conical introduction port portion extending divergently in the rearward direction from the seal region toward an opening of the stepped bore is formed, and a cone angle xcex1 of the introduction port portion with respect to the axis of the tubular member being set not larger than 35xc2x0.
When the introduction port portion of a cone angle of not larger than xcex1=35xc2x0 is formed in the stepped bore as mentioned above, the O-ring is forced into the seal region as the O-ring is deformed slowly, so that the breakage and cracking of the O-ring rarely occur. When the cone angle xcex1 is set in the range of 10xc2x0 to 20xc2x0, which structure will hereinafter be referred to as preferred aspect of the invention (2), a substantial rate of occurrence of damage to the O-ring of 0% can be attained. Preferably, the axial length of the introduction port portion is set to a level which permits the O-ring set in the introduction port portion, and not yet forced down, to be sunk by at least a half portion thereof into the introduction port portion, which structure will hereinafter be referred to as preferred aspect of the invention (3). This enables the O-ring to be smoothly forced into the stepped bore.